Happy people are a lot more fun to be around and consequently have better relations at work. This translates into:
• Better teamwork with your colleagues
• Better employee relations if you’re a manager
• More satisfied customers if you’re in a service job
• Improved sales if you’re a sales person

Herz also points out that there's scientific backing for these conclusions. Referencing blogger Kathy Sierra, he writes:

due to the phenomenon of emotional contagion, “negative emotions exert a more powerful effect in social situations than positive ones.” That’s why a generally happy and upbeat person will likely become depressed or angry when hanging out with someone who’s depressed or angry. According to a source Sierra quotes, the converse is also true: If we’re around someone who’s self-confident and buoyant long enough, we’re likely to feel good about ourselves.

Happy people are a lot more fun to be around and consequently have better relations at work. This translates into:
• Better teamwork with your colleagues
• Better employee relations if you’re a manager
• More satisfied customers if you’re in a service job
• Improved sales if you’re a sales person

Herz also points out that there's scientific backing for these conclusions. Referencing blogger Kathy Sierra, he writes:

due to the phenomenon of emotional contagion, “negative emotions exert a more powerful effect in social situations than positive ones.” That’s why a generally happy and upbeat person will likely become depressed or angry when hanging out with someone who’s depressed or angry. According to a source Sierra quotes, the converse is also true: If we’re around someone who’s self-confident and buoyant long enough, we’re likely to feel good about ourselves.